1. Field of the Invention
The present invention relates generally to telecommunication systems, and more specifically, to a method for serving character message transmission during telephone calls using voice channel over PSTN (Public Switched Telephone Network).
2. Description of the Related Art
E-mail (electronic mail) service or SMS (Short Message Service) is available to the vast majority of terminals like computers or cellular phones. To keep abreast of such a communication environment, more people now want to get the e-mail service and SMS using their wire telephones at homes and in offices. Therefore, transmitting/receiving data over the general wire telephone networks using voice channels is no longer a dream. In fact, a number of people are already enjoying the caller ID (hereinafter, referred to as “CID”, caller identity or caller identification) display service, and short message service (hereinafter, referred to as “SMS”) by transmitting/receiving data through voice channels over the general wired telephone networks. Through CID display service, a terminating party (receiving party) is provided with information about a caller (name, calling time, caller's phone number) before picking up the phone, or about another caller who made a call while the terminating party being in the middle of conversation on the phone with a different person. On the other hand, SMS service enables any PSTN (Public Switched Telephone Network) subscriber or mobile communication subscriber to transceive an SMS message, e-mail with other Internet users, and facsimile messages. Data transmission through voice channel in such services is done by using FSK (frequency shift keying) method.
More specifically, CID display service is provided in conforming to the following procedure. In case that the CID display service is provided before a caller and a called party, talk on the phone, the central office provides CID of the caller to the called party between the first ring signal and the second ring signal. Meanwhile, if the CID display service is provided in the middle of the conversation between the caller and the called party, that is, a third party calls, the central office first transports CAS (CPE (Customer premise equipment) Alerting Signal) to the called party, and receives ACK (Acknowledgement) from the called party, and transports the CID for the third party to the called party.
Next, SMS is provided in conforming to the following procedure. To transmit SMS message from a terminal, a subscriber needs to dial a PSMSC (PSTN Short Message Service Center) number, a server for providing SMS message service, and connects to the PSMSC. In this way, the voice channel between the terminal and the PSMSC is successfully connected. Then, PSMSC transmits CAS to the terminal, and the terminal, having received the CAS, responds as ACK. In this manner, data transmission is initialized. Afterward, the terminal transports SMS message to PSMSC. Transmission of the SMS message from a terminal is almost the same with the above procedure except that the PSMSC's phone number is included in the CID of the call. The PSMSC's phone number is necessary for automatic termination. After the voice channel is connected with PSMSC, data transmission is initialized, and the PSMSC receives the SMS message that the terminal transmitted. Here, the data transmission protocol and electrical characteristic associated with the CID display service and SMS conform to short message send-receive interface technical regulation over the wire network and to data transmission interface technical regulation in PSTN (Public Switched Telephone Network) subscriber line for the CID display, which are established by Korea Telecom Inc.
Nevertheless, CID display services and SMS in which data can be transceived using voice channels over the general wire telephone network are possible only through a server like the PSMSC. Therefore, the services are not available when a caller and a called party want to communicate using characters in the middle of their conversation over the general wire telephone network.